1. Field of the Invention
This invention relates generally to a drive circuit for a piezo-electric transducer, such as, a bi-morph leaf, and more particularly is directed to such bi-morph leaf and the drive circuit therefor in an apparatus for reproducing video or other information signals recorded in a track on a record medium, for example, as in a video tape recorder, and in which a magnetic head carried by the bi-morph leaf is made to accurately scan the track in response to a drive voltage applied by the drive circuit to the bi-morph leaf.
2. Description of the Prior Art
It is known to bond together a pair of piezo-ceramic elements to form a bi-morph leaf, and to apply a deflection voltage to the elements so as to cause them to bend or deflect along a lengthwise direction of the leaf. The individual ceramic elements which make up the bi-morph leaf are made of a high-dielectric constant polycrystalline material which acquires piezo-electric properties when it is poled or polarized by being subjected to a strong unidirectional voltage.
If a pair of electrically poled piezo-ceramic elements interposed between inner and outer electrodes are bonded to each other at the respective inner electrodes, and a voltage is applied between the bonded-together inner electrodes and the outer electrode of each element such that the voltage is in the poling direction of one element and opposed to the poling direction of the other element, the bending effect is multiplied in respect to that obtainable with only a single piezo-ceramic element. However, in any case where a large amount of bending is required, correspondingly large deflection voltages have to be applied. Such large deflection voltage, when applied in a direction opposed to the poling direction of a piezo-ceramic or -electric element, tends to depolarize the element and reduce its future ability to bend or deflect.
One use of a bi-morph leaf requiring large deflections thereof is in supporting a magnetic head in a video tape recorder or VTR so that, as the magnetic head is moved along a track on a record medium in which video or other information signals are required, a drive voltage or signal can be applied to the bi-morph leaf for deflecting the head in a direction transverse to the length of the track, for example, so as to maintain optimum positioning of the head in respect to the record track.
Arrangements have been proposed to provide relatively large amplitude, bidirectional deflections of a bi-morph leaf without depolarizing the latter, for example, as disclosed in the U.S. patent applications of Raymond Francis Ravizza, identified as Ser. No. 668,581, filed Mar. 19, 1976, and Ser. No. 677,828, filed Apr. 16, 1976, and both of which became known to the assignee of the present application by the opening to public inspection of a corresponding application filed on behalf of such applicant in a country foreign to the United States and in which copies of the identified U.S. applications were filed in support of a claim of priority. In such arrangements, the bonded together piezo-ceramic elements are given the same poling direction, and the deflection voltages applied to the piezo-ceramic elements are made to have polarities in the same direction as the poling direction of the elements to which the deflection voltages are applied. More particularly, a DC bias voltage is applied to each piezo-ceramic element such that the applied voltage is in the poling direction of the element, and an AC deflection voltage is superposed on the DC bias voltage for controlling the deflection of the bi-morph leaf, with the magnitude of the bias voltage being sufficiently large to ensure that the net voltage applied to each piezo-ceramic element always has a polarity in the poling direction of that element.
The foregoing known arrangement for avoiding depolarizing of a bi-morph leaf is disadvantageous, particularly when the bi-morph leaf is employed for the mounting of a magnetic head on a rotary portion of a guide drum in a video tape recorder of VTR. More particularly, in such use of a bi-morph leaf, drive voltages or signals for deflecting the bi-morph leaf and thereby moving the magnetic head transversely in respect to the record track are applied through two drive amplifiers and respective bias sources to the outer electrodes, respectively, of the piezo-ceramic elements which have their bonded-together inner electrodes connected to ground. Thus, apart from the undesirable cost inherent in providing the two drive amplifiers and associated bias sources, the described arrangement according to the prior art requires a somewhat complicated structure for electrically insulating the magnetic head from the bi-morph leaf. Further, two slip rings are required for transmitting the drive voltages from the sources thereof to the rotary portion of the guide drum. Moreover, by reason of the DC bias voltages applied to the piezo-ceramic elements for avoiding the depolarizing thereof, the length of the bi-morph leaf tends to change over an extended period of time with the result that the projection of the magnetic head in respect to the peripheral surface of the guide drum is undesirably varied.